In a semiconductor power converter, a semiconductor device such as IGBT is driven by a pulse-width modulated switching signal for a desired voltage reference (hereinafter, the pulse-width modulation is abbreviated as PWM). This PWM causes an alternating current to have a waveform in which a ripple component caused by the switching is superimposed on its fundamental wave component. In order to control the current, it is preferable to detect the current by extracting only the fundamental wave component with the least effect of the ripple component. Patent Document 1 suggests a technique to detect an alternating current, using an alternating current transducer, at the point of time corresponding to an approximate midpoint between zero voltage intervals (intervals when the AC output voltage becomes zero).
In Patent Document 1, currents of three phases (U-phase, V-phase, and W-phase) are successively detected, based on the observation that the point of time when the amplitude of a triangle-wave carrier signal (a carrier wave of PWM) reaches its maximum or minimum, corresponds to an approximate midpoint between zero voltage intervals.
Patent Document 1: Japanese Patent Laid-open No. Hei 6 (1994)-189578